1. Field of the Invention
The present invention relates to a hydraulic pressure control device for an automatic transmission system, and more particularly relates to such a hydraulic pressure control device which is particularly suitable for a twin type automatic transmission system which incorporates first and second transmission mechanisms arranged in series in the rotational force transmission path.
2. Description of the Prior Art
In the prior art, there have been proposed various types of automatic transmission systems which incorporate first and second transmission mechanisms arranged in series in the rotational force transmission path, i.e. with the second transmission mechanism receiving the output of the first transmission mechanism and further transmitting it, particularly in Japanese Patent Laying Open Publications Ser. Nos. 56-138020 (1981) and 58-128929 (1983), neither of which is it intended hereby to admit as prior art to the present patent application except to the extent in any case required by applicable law.
Conventionally, in such a twin type transmission system in which the first and the second transmission mechanisms are both hydraulic type transmissions such as gear transmission mechanisms which are set to their various speed stages by selective supply of actuating hydraulic fluid pressures to various friction actuating mechanisms incorporated in them such as hydraulic clutches and hydraulic brakes, the actuating pressures which are thus selectively supplied to both the first transmission mechanism and also the second transmission mechanism are provided by switching the same line pressure by appropriate control mechanisms such as hydraulic valves. In other words, the two hydraulic fluid pressure control devices for the first and the second transmission mechanisms are conventionally both provided with the same line pressure from one and the same line pressure control valve.
There is however a problem with such a twin type transmission system, in that, in the typical case that the second transmission mechanism is provided in the power transmission path after the first transmission mechanism and thus receives a supply of rotational power from said first transmission mechanism, the torque acting upon the friction engaging mechanisms such as the hydraulic clutches and the hydraulic brakes of said second transmission mechanism is not only determined by the load on the vehicle engine, but also fluctuates depending on the engaged speed stages both of the first transmission mechanism and also of the second transmission mechanism. In other words, even if the engine load is substantially constant, when the first transmission mechanism is set to a relatively low speed stage, the torque on the friction engaging mechanisms such as the hydraulic clutches and the hydraulic brakes of said second transmission mechanism is higher than when said first transmission mechanism is set to a relatively high speed stage. Accordingly, in the above outlined conventional case that the supply of line hydraulic fluid pressure for being switched for providing the actuating hydraulic fluid pressures for the various friction engaging mechanisms of said second transmission mechanism is the same supply as the supply of line hydraulic fluid pressure for being switched for providing the actuating hydraulic fluid pressures for the various friction engaging mechanisms of said first transmission mechanism, and in the conventional case that the pressure value of said line pressure does not vary according to engagement of the speed stages of either of said transmission mechanisms (although it may vary with engine load), the problem arises that the supply of line hydraulic fluid pressure for said second transmission mechanism is sometimes unsuitable, even if it is appropriate for said first transmission mechanism. This can lead to slippage of said friction engaging mechanisms, thus deteriorating the performance of the transmission, reducing its reliability, and shortening its service life.
As a means of compensating for this problem, it has been conceived of to appropriately increase the number of plates in the friction engaging mechanisms of the second transmission mechanism, in order to increase the torque transmission capability thereof. This can certainly remove all problems relating to shortage of torque transmission capability in said friction engaging mechanisms of the second transmission mechanism, but at the price of increasing the size and weight and cost of said second transmission mechanism. Such a concept is accordingly not very economically feasible.